1. Field of the Invention
The present invention generally relates to etch chambers. More particularly, the present invention relates to a process for cleaning copper oxides from a substrate using an etch chamber.
2. Background of the Related Art
As integrated circuit (IC) dimensions become increasingly smaller, the interface contact resistance between interconnect layers becomes significantly important. A particular contact resistance problem involves metal oxides that form when the substrate is exposed to air during the fabrication process. In the past, sputter etching has been utilized successfully to remove many metal oxides. However, as device dimensions and interconnect features shrink, sputter etching has become inadequate for removing metal oxides, particularly copper oxides, within the smaller interconnect features.
In order to fabricate a complete IC, typically several substrate processing systems are used, with each system performing a particular step or series of steps in the overall fabrication process. The substrates are transferred between the systems at ambient conditions. The ambient environment is maintained very clean to prevent contamination of the substrates as they are transferred between systems. The substrates may even be transferred in completely enclosed cassettes in order to further prevent contamination thereof. A problem, however, is that oxygen in the ambient air form oxides on the surfaces of the substrates, and in the case of copper deposits, copper oxides form on the surface of the deposited copper film. The surface oxide creates an interface having a high contact resistance with a subsequently deposited metal film and degrades the device performance because of the excess interface resistance. Thus, the surface oxides need to be removed or etched from the surfaces of the substrates, in a pre-processing cleaning or pre-clean step, before the substrates are subjected to subsequent processing, in order to assure a very low interface resistance with a subsequently deposited layer.
A pre-clean chamber cleans the surface of the substrate by removing the undesired layer of oxides. FIG. 1 is a simplified cross sectional view of a pre-clean chamber. Generally, the pre-clean chamber 10 has a substrate support member 12 disposed in a chamber enclosure 14 under a quartz dome 16. The substrate support member 12 typically includes a central pedestal plate 18 disposed within a recess 20 on a quartz insulator plate 22. The upper surface of the central pedestal plate 18 typically extends above the upper surface of the quartz insulator plate 22. A gap 24, typically between about 5 mils and 15 mils, is formed between a bottom surface of the substrate 26 and the top surface of the quartz insulator plate 22. During processing, the substrate 26 is placed on the central pedestal plate 18 and located thereon by positioning pin 32. The peripheral portion of the substrate 26 extends over the quartz insulator plate 22 and overhangs the upper edge of the quartz insulator plate 22. A beveled portion 28 of the quartz insulator plate 22 is disposed below this overhanging peripheral portion of the substrate 26, and a lower annular flat surface 30 extends from the lower outer edge of the beveled portion 28. The insulator plate 22 and the dome 16 may comprise other dielectric materials, such as aluminum oxide and silicon nitride, and the insulator plate 22 and the dome 16 are typically parts of a process kit that system operators periodically replace during routine maintenance. It is desirable that a process kit has a long useful lifetime so that the downtime of the system will be a small percentage of the overall processing time.
The process for cleaning the substrate 26 in the pre-clean chamber 10 generally involves a sputter-etching process using the substrate 26 as the sputtering target. Generally, a cleaning gas such as argon is flowed through the chamber 10, and a plasma is struck in the chamber with a bias power applied to the substrate 26 in the range of about 150 W to about 450 W. Additionally, a RF power is applied to the chamber through coils disposed outside of the chamber. A DC bias of about xe2x88x92100 V to about xe2x88x92600 V, with a bias power of about 100 W to about 300 W, accelerates the ions toward the substrate 26. The pressure in the pre-clean chamber 10 during sputtering is typically between about 0.4 mTorr and about 0.5 mTorr. Under these conditions, the pre-clean chamber 10 can typically remove about 150 xc3x85 to about 450 xc3x85 of the oxidized material at an etch rate of about 300 xc3x85/min to about 600 xc3x85/min. Typically, about 400 xc3x85 or less of oxidized material is removed from the surface of the substrates.
The primary purpose of the etch cleaning is to remove the oxidized materials that form on the surface of the substrate typically when the substrate was subjected to ambient air conditions while being transported between processing chambers of a processing system or from one processing system to another processing system. For cleaning of copper oxides that form on the surface of a deposited copper film on a silicon substrate, the substrate is processed in the pre-clean chamber 10 as described above. The etched material (i.e., copper oxides) sputters off the substrate surface and forms a film on the process kit. As the film forms on the process kit surfaces, its density may change, resulting in stress on the film. This stress, along with differences in the coefficients of expansion of the materials in the film and the process kit surfaces, can result in delamination, or flaking, of the film from the surface of the process kit and contamination of the substrate being processed. Because these particles can seriously damage the substrates and/or cause defects to form on the substrates, the process kit is typically replaced after a certain number of substrates have been cleaned in the system. However, replacement of the process kit is time consuming and reduces throughput of the system. Additionally, as long as a film is formed on the surfaces of the process kit, there is a risk of flakes of material falling onto a substrate and damaging the devices formed on the substrate.
Furthermore, where the copper oxide is formed on the bottom surface of an interconnect feature, such as a contact or a via, some of the sputtered copper oxide from the bottom of the interconnect feature deposits onto the side wall of the interconnect feature. The copper from the copper oxide may diffuse through the dielectric material that forms the side wall of the interconnect feature and degrade the device performance. Also, when a subsequent layer or a barrier layer, such as tantalum (Ta) or tantalum nitride (TaN), is deposited over the surfaces within the interconnect feature where the copper oxide has deposited, the film quality of the subsequent layer is compromised or degraded by the copper oxide that sputtered onto the side walls of the interconnect feature.
Therefore, there exists a need for an apparatus and a method of removing metal oxides, particularly copper oxides, from a substrate surface that prevents sputtering of copper oxides from the bottom of an interconnect feature onto the side walls of an interconnect feature. Furthermore, there is a need to eliminate sputtering of the copper oxides onto the process kit that may eventually flake off and cause defects on the substrate.
The invention generally provides an apparatus and a method of removing metal oxides, particularly copper oxides and aluminum oxides, from a substrate surface. Primarily, the invention eliminates sputtering of copper oxide from the bottom of an interconnect feature onto the side walls of an interconnect feature, thereby preventing diffusion of the copper atom through the dielectric material and degradation of the device. The invention also eliminates sputtering of the copper oxides onto the chamber side walls that may eventually flake off and cause defects on the substrate.
The method of reducing metal oxides from a substrate surface comprises placing the substrate within a plasma processing chamber, flowing a processing gas comprising hydrogen into the chamber, and maintaining a plasma of the processing gas within the chamber through inductive coupling. Preferably, the processing gas comprises about 5% or less hydrogen premixed with an inert carrier gas, such as helium or nitrogen. The method is preferably performed using a dual frequency etch chamber wherein adjustments are made in the processing gas flow, the RF powers and the exhaust pumping speeds to eliminate sputtering of the copper oxide and to maximize the reduction reaction.